1. Field of the Invention
The present invention relates to an optical device, a control method for the optical device, and an image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus forms an image in such a manner that an optical writing device exposes an electrostatic charge formed on a photosensitive drum to a laser beam thereby forming an electrostatic latent image on the photosensitive drum, and the electrostatic latent image is developed into an image by application of developer. Conventionally, as a light source of a laser beam, a semiconductor laser element, such as a laser diode (LD), which emits one or a plurality of laser beams from one element has been known. An LD which emits a plurality of laser beams is called an LD array, and an LD array which emits four to eight laser beams is generally used in an image forming apparatus.
Furthermore, in recent years, a surface-emitting laser called “VCSEL (Vertical Cavity Surface Emitting LASER)”, which can emit a few dozens (for example, forty) laser beams from one element, has been put to practical use. Accordingly, there has been proposed an image forming apparatus which uses a VCSEL as a laser light source and is capable of forming a high-resolution image at high speed.
To perform image formation using a laser beam, a light quantity of a laser beam to illuminate a photosensitive drum has to be kept constant. A laser diode emits a laser beam in a normal direction, i.e., toward an object to be illuminated as well as a back beam of a light quantity proportional to that of the laser beam in a direction opposite to the laser beam. Conventionally, the light quantity of the laser beam emitted in the normal direction is controlled by means of APC (Auto Power Control) using this back beam.
As a specific example of the APC, a photodiode (PD) is placed as a light receiving element in the same package as a laser diode unit, and the PD receives a back beam. The PD converts the received back beam into an electric current by means of photoelectric conversion, and converts the electric current into a voltage using resistance or the like, and then measures a value of the voltage. A light quantity of the back beam is proportional to a light quantity of a laser beam emitted in the normal direction, so a value of electric current to be applied to a laser diode is controlled so that a measured voltage value is kept constant by feeding back the voltage value. This enables a light quantity of the laser beam emitted in the normal direction to be kept constant.
Here, let us think about the above-described case where one element emits a plurality of laser beams. For example, in the above-described LD array, it is necessary to cause a plurality of back beams corresponding to a plurality of laser beams to enter one PD placed in the LD array; therefore, as the number of laser beams increases, it becomes difficult to perform the APC. Furthermore, for example, in a VCSEL, there is no back beam; therefore, it is not possible to apply the APC using a back beam.
Consequently, when the APC is performed on a plurality of laser beams, there is used the following method: a portion of the laser beam is reflected by a plurality of optical components and used as a monitor beam; a light quantity of the monitor beam is measured; the measured light quantity is converted into a voltage; and a value of the voltage is fed back to a value of drive current. Hereinafter, this APC method using a monitor beam is referred to as a “front monitoring method”.
In the front monitoring method, the optical components for reflecting a portion of a laser beam and the PD for receiving a monitor beam reflected by the optical components are arranged to keep a relatively long distance from the LD array or VCSEL. Therefore, for example, when the device including these optical systems is subject to strong impact, the arrangement of the optical components and the PD may change, and an optical axis of the monitor beam with respect to the PD may be shifted, and as a result, the monitor beam may not enter the PD. If the APC is performed in a state where the monitor beam does not enter the PD, a light quantity of the monitor beam detected by the PD becomes about zero, which results in emission of a laser beam with an excess drive current, and this may cause degradation or breakdown of the LD array or VCSEL which is a light source.
Therefore, when the front monitoring method of APC is performed on a laser diode, it is necessary to provide a means of detecting misalignment of an optical axis of a laser beam with respect to a PD.
Conventionally, various technologies applicable to detection of such misalignment of an optical axis of a laser beam with respect to a PD have been proposed and put to practical use. For example, in a technology disclosed in Japanese Patent Application Laid-open No. 2002-141605, a device for measuring a value of voltage correlating with a drive current applied to a light source being subjected to the APC is provided, and the current voltage value is compared with a preset voltage value, and if the current voltage value exceeds the preset voltage value, it is determined that the light source is degraded. Furthermore, a technology disclosed in Japanese Patent Application Laid-open No. 2003-182140, if a drive current of a laser beam exceeds an upper limit of control range of drive amount during the APC, it is determined as malfunction. Moreover, a technology disclosed in Japanese Patent Application Laid-open No. 2008-74098, before the APC of a laser diode is performed, a PD-output-voltage feedback system is shut down, a laser beam is emitted with a prescribed drive current, and an output voltage from a PD at the time is checked, and only if a value of the voltage is within a prescribed value, the APC is performed.
In the technology disclosed in Japanese Patent Application Laid-open No. 2002-141605, although a method to determine degradation of a light source by monitoring a back beam is described, this method can be employed in detection of misalignment of an optical axis of a laser beam with respect to a PD in the front monitoring method. However, according to the technology disclosed in Japanese Patent Application Laid-open No. 2002-141605, abnormality in a value of voltage correlating with a drive current is detected during the APC, so even if an optical axis of a laser beam with respect to the PD is shifted, the APC is executed. Therefore, the light source is driven with a drive current based on a voltage value exceeding the preset voltage value, and this may cause degradation or breakdown of the normal LD array or VCSEL.
In Japanese Patent Application Laid-open No. 2002-141605, the preset voltage value is a value determined by taking variations of optical writing devices into account, and a largish acceptable value is generally set to the voltage value so as to prevent any optical writing devices from determining malfunction incorrectly. Therefore, even if a drive current is controlled to be within the acceptable value, an excess amount of current is likely to be supplied to the LD array or VCSEL, and there is a high possibility of causing degradation or breakdown of the LD array or VCSEL.
Also in the technology disclosed in Japanese Patent Application Laid-open No. 2003-182140, in the same manner as Japanese Patent Application Laid-open No. 2002-141605, even if an optical axis of a laser beam with respect to a PD is shifted, the APC is executed at least once. In this case, the LD array or VCSEL is driven with a drive current which is out of a predetermined range, so there is a possibility of causing degradation or breakdown of the normal LD array or VCSEL.
On the other hand, according to the technology disclosed in Japanese Patent Application Laid-open No. 2008-74098, before the APC is performed on the LD, the PD-output-voltage feedback system is shut down and an output voltage from the PD is checked, and the APC is performed if a value of the voltage is within a prescribed range; therefore, it is possible to prevent degradation or breakdown of the LD array or VCSEL due to the APC like in Japanese Patent Application Laid-open No. 2002-141605.
However, in general, a laser light source, such as an LD array or a VCSEL, varies greatly in a quantity of light emitted according to an amount of individual drive current; therefore, when the laser light source emits laser beams with a prescribed drive current, a light quantity of the emitted laser beams runs over a wide range in each device, and a prescribed range of voltage from the PD to be determined before the APC is performed has to be set to a wide range. Therefore, when the method according to Japanese Patent Application Laid-open No. 2008-74098 is applied to detection of misalignment of an optical axis of a monitor beam with respect to the PD, it is not possible to expect high-accuracy detection.
Furthermore, besides misalignment of an optical axis of a monitor beam with respect to the PD, a decrease in output voltage from the PD may occur when a light quantity of a laser beam extremely drops due to degradation of the LD array or VCSEL provided as a light source or when no laser beam is emitted due to breakdown of the LD array or VCSEL. In the method disclosed in Japanese Patent Application Laid-open No. 2008-74098, when a decrease in output voltage from the PD is confirmed, it is not possible to determine whether the decrease in output voltage arises from misalignment of an optical axis of a monitor beam with respect to the PD. Therefore, when a decrease in output voltage from the PD is confirmed, both the PD and the light source have to be replaced.
An LD array and a VCSEL are very expensive as compared with an ordinary semiconductor laser; thus, breakdown of the normal light source is caused by performing the APC in a state where there is optical-axis misalignment, which further causes a negative effect of an increase in servicing or maintenance cost. Therefore, to employ the front monitoring method of APC, a method capable of detecting misalignment of an optical axis of a laser beam with respect to a PD with a high degree of accuracy is required.